PROPOSAL (Adapted from the applicant's abstract): IE is one of the most serious medical complications associated with intravenous drug use, with significant morbidity and mortality occurring in unrecognized cases. Presenting clinical features associated with IE have been difficult to define. Since fever occurs in nearly 98% of cases of IE in IDUs, current practice calls for hospitalization of all febrile IDUs for further evaluation. The overall goal of this study is to develop and implement a safe and cost- effective emergency department (ED)-based algorithm for the evaluation of febrile IDUs at risk for IE. The investigators hypothesize that, by taking advantage of recent developments which combine findings from advanced diagnostic testing (including echocardiography) and clinical assessment, this algorithm will enable ED physicians to more quickly and reliably stratify patients at risk for IE. They further hypothesize that disposition of the subset of patients in whom ED has been ruled out to less resource-intensive settings will reduce the number of hospitalizations without increasing adverse events, thus promoting more efficient use of clinical resources. To test these hypotheses, they propose a program consisting of three specific aims: 1) To derive an algorithm that will allow ED physicians to predict with increased speed and specificity which febrile IDUs are likely to either have IE or develop IE-related complications that require hospitalization. Standard clinical, laboratory and imaging information as well as a novel laboratory- based diagnostic tool will be utilized to derive the algorithm. 2) To determine the 'opportunity costs' (i.e., the value of resources that would be recovered and available for other uses) associated with the current practice of universal hospitalization. 3) To implement and conduct a comprehensive cost effectiveness analysis of the derived clinical algorithm. The study will employ a prospective, two-phase design. In the first phase, alternative models will be compared and evaluated among patients treated according to the standard practice of universal hospitalization. In the second phase, patients will be evaluated and discharged to three possible settings: home with follow-up, a simulated ED observational unit, or hospitalization, as indicated by the clinical algorithm. They will estimate the impact of the algorithm on costs and patients' outcomes. Findings from this research will provide valuable information that can be used to guide and improve evaluation and management of the growing number of febrile IDUs.